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Technology: The flight of the powerless aeroplane

By PETER HADFIELD

A model aeroplane with no source of power on board will soon make its
maiden flight, say Japanese researchers. Instead of carrying fuel or batteries,
power is beamed up to the 1.6-metre plane as microwaves which it converts
into electricity to drive its propeller.

Microwaves were first used to power a plane in Ottawa, Canada, in 1987,
but the Japanese have refined the system taking advantage of advances in
semiconductor technology. Their aim, however, is not to produce airliners
without fuel tanks but to test the technology necessary for orbiting solar
power stations. Satellites carrying huge arrays of solar panels could beam
the energy they gather to other satellites or down to Earth as microwaves.

‘The Canadian plane depended on a dish antenna on the ground for microwave
transmission, which had to be manually controlled,’ says Hiroshi Matsumoto
of Kyoto University’s Radio Atmospheric Science Centre. ‘We are planning
to use a phased array antenna.’

Such antennas consist of a fixed array of hundreds of small transmitters.
By slightly delaying the microwave signal from some transmitters with respect
to others, the combined beam can be ‘steered’ in different directions.

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With a phased array antenna the microwave beam can be moved faster and
pointed more accurately. This is important for the researchers’ solar power
station plans. ‘We have to control things electronically – it can’t be done
manually,’ says Matsumoto.

The phased array transmitter now being readied for the test flight in
June will, if all goes well, be launched into space in 1993. From an altitude
of 220 kilometres, it will beam microwaves to a ‘daughter’ satellite launched
at the same time.

‘I believe a power-generating satellite will one day be needed to power
small-scale factories in space,’ says Matsumoto, ‘but there are also spin-offs
on the ground.’ One of these, now being studied by Japan’s Ministry of Post
and Telecommunications, is to use pilotless aeroplanes, powered by microwave
beams, to relay radio signals for mobile communications.